The present invention relates to a controller for a magnetic bearing which is used as a radial bearing or a thrust bearing in turbomachinery, that is, a magnetic bearing of the type having a rotor yoke rigidly secured to a rotary shaft, a pair of electromagnet stators rigidly secured to a casing with a minute gap provided between the same and the rotor yoke, each stator being provided with an exciting coil for generating magnetomotive force, and a displacement sensor for measuring relative displacement between the rotary shaft and the casing, wherein magnetic attraction is caused to act between the rotor yoke and the pair of electromagnet stators on the basis of a signal output from the displacement sensor, thereby supporting the rotary shaft in free space.
A magnetic bearing system having a controller according to a prior art will first be explained with reference to FIGS. 3 and 4.
FIG. 3 is a vertical sectional view showing the structure of a spindle which is supported by a conventional quinaxial control magnetic bearing system that employs electromagnets (i.e., one type of bearing system wherein the motion of the spindle is controlled in five different directions).
Referring to FIG. 3, a rotary shift 1 is driven by an electric motor which has a motor stator 8 and a motor rotor 9 which are disposed in the central portion of a casing 7, and the rotary shaft 1 is supported by two radial magnetic bearings disposed at both sides, respectively, of the motor and a thrust magnetic bearing which is adjacent to one of the radial magnetic bearings. Each of the radial magnetic bearings comprises a pair of radial bearing stators 3 rigidly secured to the casing 7 and each is provided with a stator coil 5, a radial bearing rotor yoke 4 secured to the rotary shaft 1, and a radial displacement sensor 6. The thrust magnetic bearing comprises a thrust bearing rotor 10 secured to the rotary shaft 1 and a pair of thrust bearing stators 11 secured to the casing 7 in such a manner that the stators 11 face each other across the rotor 10, each stator being provided with a stator coil 12, and an axial displacement sensor 13 provided at the end of the rotary shaft 1. The reference numeral 2 denotes an emergency rolling bearing.
Although two different kinds of bearing, that is, the radial bearings each comprising the bearing members 3, 4, 5 and 6 and the thrust bearing comprising the bearing members 10, 11, 12 and 13, are shown in FIG. 3, an explanation will be given in regard to the radial bearings alone since these two kinds of bearing may be controlled by the same control method.
In each radial bearing, the displacement sensor 6 measures relative displacement between the bearing rotor yoke 4 secured to the rotary shaft 1 and the bearing stators 3 each provided with a coil 5 for generating magnetomotive force, and a control current is applied to each coil 5 so that the distance between the yoke 4 and the stators 3 is maintained at a constant level.
FIG. 4 is a circuit diagram of a conventional controller for controlling the supply of current to the above-described coils 5. Referring to the figure, a signal output from the displacement sensor 6 is passed through a compensation circuit 21 and a power amplifier 22 to supply a control circuit to the stator coils 5e and 5f and, at the same time, a direct current from a DC power supply 23 is constantly supplied to the stator coils 5c and 5d to ensure linearity for control purposes.
Thus, in the conventional magnetic bearing controller, a relatively large current is constantly supplied to the stator coils from a DC power supply in order to realize linear control, and this leads to various problems, that is, generation of heat in the coil, power loss in the control system, generation of heat in the rotor due to eddy-current loss in the rotor, and unstable vibration of the rotary shaft.